GPS, or Global Positioning System, is funded by and controlled by the U.S. Department of Defense (DOD). While there are many thousands of civil users of GPS world-wide, the system was designed for and is operated by the U.S. military. GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity, and time. Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock. The GPS satellites transmit two microwave carrier signals. The L1 frequency (1575.42 MHz) carries the navigation message and the Satellite Positioning Service (SPS) code signals. The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay by Precise Positioning System (PPS) equipped receivers.
Three binary codes shift the L1 or L2 carrier phase. The C/A Code (Coarse Acquisition) modulates the L1 carrier phase. The C/A code is a repeating 1 MHz Pseudo Random Noise (PRN) Code. This code modulates the L1 carrier signal, spreading the spectrum over a 1 MHz bandwidth. The C/A code repeats every 1023 bits (one millisecond). Each satellite has a different PRN C/A code, and GPS satellites are often identified by their PRN number, the unique identifier for each pseudo-random-noise code. The C/A code that modulates the L1 carrier is the basis for the civil SPS.
Authorized users with cryptographic equipment and keys and specially equipped receivers use the Precise Positioning System, or PPS. Authorized users include U.S. and allied military, certain U.S. Government agencies, and selected civil users specifically approved by the U.S. Government. In the PPS, the P-Code (Precise) modulates both the L1 and L2 carrier phases. The P-Code is a very long (seven days) 10 MHz PRN code. In the Anti-Spoofing (AS) mode of operation, the P-Code is encrypted into the Y-Code. The encrypted Y-Code requires a classified AS Module for each receiver channel and is for use only by authorized users with cryptographic keys. The P/Y Code is the basis for the PPS.
A Navigation Message also modulates the L1-C/A code signal. The Navigation Message is a 50 Hz signal consisting of data bits that describe the GPS satellite orbits, clock corrections, and other system parameters. The GPS Navigation Message consists of time-tagged data bits marking the time of transmission of each subframe at the time they are transmitted by the SV. A data bit frame consists of 1500 bits divided into five 300-bit subframes. A data frame is transmitted every thirty seconds. Three six-second subframes contain orbital and clock data. Satellite Vehicle (SV) Clock corrections are sent in subframe one and precise satellite orbital data sets (ephemeris data parameters) for the transmitting SV are sent in subframes two and three. Subframes four and five are used to transmit different pages of system data. An entire set of twenty-five frames (125 subframes) makes up the complete Navigation Message that is sent over a 12.5 minute period.
Ephemeris data parameters describe SV orbits for short sections of the satellite orbits. Normally, a receiver gathers new ephemeris data each hour, but can use old data for up to four hours without much error. The ephemeris parameters are used with an algorithm that computes the SV position for any time within the period of the orbit described by the ephemeris parameter set.
The C/A code is broadcast at 1,575.42 MHz in a 2.046 MHz wide band (complete null to null), and is used for civilian operations and for initial acquisition in military operations. The P/Y code is a wider-band signal spanning 20.46 MHz that provides 10 times higher ranging precision than C/A code commensurate with its higher chipping rate. Often, C/A code is the first casualty of jamming. The 1.023 MHz chipping rate of the C/A code provides some protection, but the 10.23 MHz chipping rate of the P/Y code offers an additional 10 dB of J/S protection. If the jamming is known to be narrow band and to originate within the C/A code frequency band so as to deny enemy use of the C/A code signal component, then even more protection is available by notch filtering the center 2 MHz of the P/Y code input to the receiver.
The typical acquisition sequence for a military receiver is to lock on and acquire with the C/A code to establish an approximate timing fix. Since the C/A code repeats every millisecond, it is usually possible to search through all possible code phases within a second or so. Once the GPS receiver clock is known, the receiver can acquire the Y code—the secure version of the P/Y code.
Unlike the C/A code, the Y code does not have the repeating quality that would make it easy to acquire. Therefore, the user receiver must have nearly exact knowledge of time before it is possible to lock onto the military Y code signal. In other words, although it may be possible to track the Y code in jammed conditions due to its wider spreading characteristics, under ordinary circumstances it may not be possible to acquire Y code because the C/A code is jammed.
Several previous solutions to the timing initialization problem have been developed and fielded, although they each suffer significant shortfalls that prevent them from realizing the full potential of GPS Y code.
The first is brute force Y code search schemes that pool all onboard hardware resources onto a single satellite in an attempt to expand the search window of unknown GPS time. This technique works well if a GPS receiver has been off for only a short time and the satellite that it selects for searching does not happen to be blocked. But under stressed conditions, it is not assured that it will lock on at all if its clock is too far off or if its search algorithm just happens to be out of synchronization with the prevailing environmental conditions. There is no graceful degradation in this case—only failure to lock.
Another method is to combine a receiver with an accurate time standard, such as an atomic clock. While this may be practical under some circumstances, this method subjects the overall system to additional costs or operational constraints. Rugged atomic standards generally cost more than the GPS receiver itself. For ground operations it is possible to hook up GPS receiver field equipment that utilizes an atomic clock that accompanies a GPS receiver or a local time-transfer facility that operates by physically interfacing a handheld GPS receiver to the timing source or some intermediate device coupled electronically to an atomic clock. Such arrangements, while technically feasible, impose unwarranted operational burdens on troops in a hostile environment. It may be impractical or cumbersome for ground troops or aircraft to take the additional step of physically connecting to an initialization device in an urgent situation or to have to rely on the assumption of ubiquitous ground support infrastructure.
Prior systems have used LEO satellite systems to augment GPS positioning. For example, U.S. Pat. No. 5,812,961 (which is incorporated by reference) describes the use of the angular velocity and position of a LEO satellite to calculate a location vector for a user. The location vector is derived by obtaining and combining user-reference carrier phases for both LEO and GPS satellites. But the '961 patent does not suggest the use of a LEO satellite to assist in acquiring a GPS clock.
What is needed is a low-cost, stand-alone GPS set that can initialize instantaneously anywhere in the world under C/A code jamming conditions.